Printed circuits are commonly made by depositing a resist on a substrate either in the form of the desired pattern or as an overall covering followed by removal of some resist to form the desired pattern, followed by modification of the bare adjacent areas of the substrate through etching or plating.
Conventional printing methods such as letterpress, lithography, and gravure printing have been found to be deficient for resist printing, however, because they are only capable of printing a thin resist. Thin resist patterns tend to be full of pinholes which lead to unacceptable quality upon subsequent etching or plating. This is a particularly severe problem in plating because of the formation of plating nodules over pinholes in the resist. Use of liquid photoresists presents the same problem.
Two methods are now in commercial use--screen printing and photoprinting--because they are able to deposit pinhole-free resist patterns. Photoprinting, as described in Celeste U.S. Pat. No. 3,469,982, requires the lamination and subsequent exposure and development of each substrate with a suitable photopolymer. While this process provides the highest quality resists and has many advantages, the expense of the materials and exposure and development steps detract from low cost rapid reproduction. Screen printing is low in ink cost but it requires a costly set-up for the master; furthermore, it has only been implemented as a flat-bed process requiring extensive operator interaction to maintain registration and correct ink viscosity. The screening also limits edge definition. Further, the process requires post-curing.
Attempts have been made to apply xerography (electrophotographic printing or imaging by electro-statically-held toner) to the resist art. By way of background in the xerography art, thermal transfer of electrostatic toner to paper has been practiced in the past. Generally, the heat was applied after the transfer of the toner, as described in U.S. Pat. Nos. 2,990,278; 3,013,027; 3,762,994; 3,851,964 and 4,015,027. Simultaneous heating and transfer of electrostatic toner to paper is disclosed in U.S. Pat. No. 3,592,642. U.S. Pat. No. 2,917,460 discloses the melting of the electrostatic toner on the paper surface so that the molten droplets so formed may be absorbed in the interstices of the paper to make a permanent image on the paper.
The above-described prior art is directed to porous substrate surfaces having substantial interstices.
As applied to the resist art, however, xerography has taken a different approach from thermal transfer. U.S. Pat. No. 2,947,625 discloses formation of an electrostatically-held image of toner, transfer of this image to a wet gelatin-coated paper using pressure which imbeds the toner in the gelatin coating, and exposing the toner image to the softening action of solvent vapors, and pressing the solvent vapor-softened toner image against a printed circuit board to transfer a stratum of the resultant tacky image to the board, and finally subjecting the transferred image to more solvent vapors or heat to coalesce the image, which is then purportedly available as an etching resist. U.S. Pat. No. 3,061,911 discloses a similar process except that the image is transferred from the transfer paper to the circuit board by electrical charging and the resultant transferred image is fused by exposure to solvent vapor. A transfer process has been commercialized, with only limited success, involving electrostatic transfer of an image of electrostatically held toner to a tissue, electrostatically transferring the image from this tissue to a circuit board, and fusing the image with solvent vapor.
Thus, thermal transfer has heretofore not been used for printing resists by xerography. In the magnetic printing art, U.S. Pat. Nos. 3,052,564 and 3,965,478 disclose that pressure transfer of an image of magnetic toner to paper can also be carried out in the optional presence of heat. The former patent discloses that heat application during transfer can reduce the transfer pressure while the latter discloses that preheating of the paper can improve adhesion of the transferred toner to paper. U.S. Pat. No. 4,067,018 discloses that in order to get a high quality image on unheated paper, free of smearing or smudging, that one or at most 11/2 layers of magnetic toner particles should be adhered to the magnetic imaging member.
As applied to the circuit making art, U.S. Pat. No. 3,880,689 discloses the magnetic printing of catalyst-sensitized toner particles in a circuit pattern onto an adhesively-coated film, followed by electroless plating of the circuit pattern to form a printed circuit. This patent also discloses that the image can be printed onto a circuit board, but this would have the disadvantage of the existence of a toner layer between the electroless plating and the circuit board. U.S. Pat. No. 3,120,806 discloses the use of a magnetic pattern placed beneath a circuit board to attract fusible metal toner to the circuit board in the pattern of the magnetic pattern, to form the circuit directly therefrom.
As applied to the resist art, U.S. Pat. No. 3,650,860 discloses a process for using magnetic toner to make a resist image. In this process, a magnetizable layer is deposited on the conductive metal substrate and this layer is imagewise heated above its Curie temperature to form a latent magnetic image in the layer. This is followed by applying a dispersion of a magnetic toner, made of ferromagnetic material dispersed in binder, in a solvent for the binder to the latent magnetic image and drying the dispersion, which thereby forms a resist image corresponding to the latent magnetic image. The bare portion of the magnetizable layer and corresponding underlying conductive metal substrate can then be etched away to form a printed circuit of the remaining conductive metal substrate. Among the disadvantages of this process is the consumption of the magnetizable layer for each printed circuit made and the necessity to use solvent to convert the magnetic toner to a liquid medium and subsequent evaporation of the solvent.